Best I can come up with is binning. Maybe some CPU's fail on the ECC front, so they sell those as 7700 and up the factory boost clock to compensate and 'market to consumers'.
Or maybe the MSRPs vary wildly, but in practice prices converged?
Or maybe the MSRPs vary wildly, but in practice prices converged?
Note that the ECC on the processor's internal data buses and caches is completely independent of whatever ECC the memory controller may or may not use.
Bingo.